Most high performance AC motor drive systems today utilize phase current sensors. Phase current information is used for controlling the machine stator currents, which in turn indirectly control machine torque. Failure of a current sensor usually results in loss of control and shutdown of the AC motor drive system.
Recently, fault tolerant control of AC motor drives has been receiving attention in the literature due to increasing application of AC drives in the automotive industry. For example, Raymond Sepe, Jr. (“Fault Tolerant Operation of Induction Motor Drives with Automatic Controller Reconfiguration”, IEMDC 2001, which is hereby incorporated by reference in its entirety) addressed current sensor faults of the induction machine type drive. In the case of current sensor failure, the drive is reconfigured from indirect field-oriented control (IFOC) to volts/Hz scalar control. Although this approach may be suitable for asynchronous induction machine drives, it is not applicable to permanent magnet (PM) type synchronous machine drives.
Field oriented control schemes are the industry standard in high performance AC drives today. Field oriented control relies on synchronous frame current regulators to correctly control machine torque. Current information is most often obtained by sensing two of the three stator phase currents. Only two sensors are needed for a machine because the machine is presumed to have balanced three-phase currents. The third current is simply calculated from the two measured currents.
In the case of a current sensor failure, the machine currents become unregulated. Usually, current will become excessive and cause an inverter to enter a fault mode that shuts down the drive. Without current sensor information, a conventional drive system is unable to resume operation.